Polyolefins have been used extensively in a wide variety of applications inclusive of food packaging, textiles, and resin materials for various molded articles. Different polymer properties may be desired depending on the intended use of the polymer. For example, polyolefins having relatively low molecular weights and narrow molecular weight distributions are suitable for articles molded by an injection molding method. On the other hand, polyolefins having relatively high molecular weights and broad molecular weight distributions are suitable for articles molded by blow molding or inflation molding. For example, in many applications, medium-to-high molecular weight polyethylenes are desirable. Such polyethylenes have sufficient strength for applications which require such strength (e.g., pipe applications), and simultaneously possess good processing characteristics. Similarly, polyolefins having a particular flow index or within a particular flow index range are suitable for various applications.
Ethylene polymers having broad molecular weight distributions can be obtained by use of a chromium-based catalyst obtained by calcining a chromium compound carried on an inorganic oxide carrier in a non-reducing atmosphere to activate it such that, for example, at least a portion of the carried chromium atoms is converted to hexavalent chromium atoms (Cr+6). This type of catalyst is commonly referred to in the art as the Phillips catalyst. The chromium compound is impregnated onto silica, dried to a free-flowing solid, and heated in the presence of oxygen to about 400° C.-860° C., converting most or all of the chromium from the +3 to the +6 oxidation state.
Another chromium-based catalyst used for high density polyethylene applications consists of silyl chromate (e.g., bis-triphenylsilyl chromate) chemisorbed on dehydrated silica and subsequently reduced with diethylaluminum ethoxide (DEAlE). The resulting polyethylenes produced by each of these catalysts are different with respect to some important properties. Chromium oxide-on-silica catalysts have good productivity (g PE/g catalyst), also measured by activity (g PE/g catalyst-hr) but often produce polyethylenes with molecular weight distributions narrower than that desired for applications such as large part blow molding, film, and pressure pipe. Silyl chromate-based catalysts produce polyethylenes with desirable molecular weight characteristics (broader molecular weight distribution with a high molecular weight shoulder on molecular weight distribution curve), but often may not have as high productivity or activity as chromium oxide-on-silica catalysts.
Monoi et al., in Japanese Patent Application 2002-020412, disclose the use of inorganic oxide-supported Cr+6-containing solid components (A) prepared by activating under non-reducing conditions, then adding dialkylaluminum functional group-containing alkoxides (B) which contain an Al—O—C—X functional group in which X is either an oxygen or a nitrogen atom, and trialkylaluminum (C) to polymerize ethylene. The resulting ethylene polymers are said to possess good environmental stress crack resistance and good blow molding creep resistance.
Monoi et al., in U.S. Pat. No. 6,326,443, disclose the preparation of a polyethylene polymerization catalyst using a chromium compound, adding an organic aluminum compound more rapidly than specified by a certain mathematical formula, and drying the resulting product at a temperature not higher than 60° C. more rapidly than specified by another mathematical formula. Both formulae are expressed as functions of batch size. Monoi teaches that by minimizing the addition time of the organic aluminum compound and the drying time, a catalyst with high activity and good hydrogen response is obtained.
Monoi et al., in U.S. Pat. No. 6,646,069, disclose a method of ethylene polymerization in co-presence of hydrogen using a trialkylaluminum compound-carried chromium-based catalyst, wherein the chromium-based catalyst is obtained by activating a chromium compound carried on an inorganic oxide carrier by calcination in a non-reducing atmosphere to convert chromium atoms into the +6 state, treating the resulting substance with a trialkylaluminum compound in an inert hydrocarbon solvent, and then removing the solvent.
Hasebe et al., in Japanese Patent Publication 2001-294612, disclose catalysts containing inorganic oxide-supported chromium compounds calcined at 300° C.-1100° C. in a non-reducing atmosphere, R3-nAlLn (R═C1-C12 alkyl; L=C1-C8 alkoxy or phenoxy; and 0<n<1), and Lewis base organic compounds. The catalysts are said to produce polyolefins with high molecular weight and narrow molecular weight distribution.
Da et al, in Chinese Patent 1214344, teach a supported chromium-based catalyst for gas-phase polymerization of ethylene prepared by impregnating an inorganic oxide support having hydroxyl group on the surface with an inorganic chromium compound aqueous solution; drying in air; activating the particles in an oxygen-containing atmosphere; and reducing the activated catalyst intermediate with an organic aluminum compound.
Durand et al., in U.S. Pat. No. 5,075,395, teach a process for elimination of the induction period in the polymerization of ethylene by bringing ethylene in contact under fluidized-bed polymerization conditions and/or stirred mechanically, with a charge powder in the presence of a catalyst comprising a chromium oxide compound associated with a granular support and activated by thermal treatment, this catalyst being used in the form of a prepolymer. The Durand process is characterized in that the charge powder employed is previously subjected to a treatment by contacting the charge powder with an organoaluminum compound in such a way that the polymerization starts up immediately after the contacting of the ethylene with the charge powder in the presence of the prepolymer.
The above described chromium-based catalysts may be used to produce select grades of polymers. Very often, polymerization reactors are required to produce a broad range of products, having flow indices that may vary from 0.1 dg/min to about 100 dg/min.
Thus, there currently exists a need for processes that allow increasing and decreasing the flow index response beyond the inherent process range of a given chromium-based catalyst. There exists a need for processes that produce chromium-based catalysts with adjustable, but predictable, flow index responses. The flow index response of a chromium-based catalyst refers to the range of the flow index of the polymer made by the catalyst under a given set of polymerization conditions.